Blends of isotactic polypropylene with elastomeric ethylene propylene copolymers are well known and are commercially significant in view of their useful properties in the production of, for example, molded parts, films, fibers and fabrics. Of particular interest are polymer blends in which the ethylene propylene copolymer is produced using metallocene catalysts since these copolymers can exhibit improved properties, such as elasticity, flexural modulus and tensile strength. For example, U.S. Pat. No. 6,642,316 discloses a polymer blend comprising from about 35% to about 85% of isotactic polypropylene and 30% to about 70% of a copolymer of propylene with about 5% to about 35% by weight of an alpha-olefin, especially ethylene, produced using a chiral metallocene catalyst. The individual components of the blend are produced separately and are then blended by any procedure that guarantees an intimate mixture of the components, for example melt pressing, melt mixing and extrusion blending.
Various processes have been proposed for producing polymer blends by multistage polymerization wherein the individual components of the blend are formed in sequential polymerization steps in the same or different reactors. In this way, the need for a separate blending step can be obviated or minimized. However, none of the existing proposals have proved entirely satisfactory and hence there is a need to provide an improved multistage polymerization process for producing polymer blends.
Today, isotactic polypropylene is widely produced commercially by a slurry polymerization process, whereas ethylene propylene copolymer elastomers are produced via solution polymerization. However, some solution polymerization processes have disadvantages in that monomer conversion is limited and the solvent must be recycled and purified. In addition, there is a limit to the molecular weight of the copolymers that can be produced, due to the resulting increase in solution viscosity. Ethylene propylene copolymer elastomers have been difficult to produce using slurry-based polymerization systems since, even at low reactor temperatures, these tend to result in reactor fouling and the formation of rubbery clumps that attach themselves to the reactor agitator, thereby necessitating reactor shut-down. Accordingly, a widely used polymerization scheme for producing reactor blends of propylene and ethylene propylene copolymer elastomers is to make the polypropylene in a slurry polymerization reactor and the copolymer elastomer in a gas phase reactor. This approach is the current basis for the production of a substantial amount of commercial polypropylene impact copolymer.
U.S. Pat. No. 6,472,474 discloses a propylene impact copolymer composition comprising from 40% to 95% by weight of Component A and from 5% to 60% by weight of Component B based on the total weight of the impact copolymer, Component A comprising a propylene homopolymer or copolymer, wherein the copolymer comprises 10% or less by weight ethylene, butene, hexene or octene comonomer, and the amount of amorphous polypropylene in Component A is less than 2 wt %; Component B comprising a propylene copolymer, wherein the copolymer comprises from 20% to 70% by weight ethylene, butene, hexene and/or octene comonomer; wherein Component B is formed in the presence of Component A in a multiple stage polymerization process. According to column 5, line 59 to column 6, line 4 of the '474 patent, each stage of the polymerization process may be independently carried out in either the gas or liquid slurry phase, although preferably Component A is polymerized in a first liquid slurry or solution polymerization reactor and Component B is polymerized in a second, gas phase reactor.
U.S. Patent Application Publication No. 2001/0039314, published Nov. 8, 2001, discloses a film comprising crystalline isotactic propylene polymer comprising: a) from 10 to 90 wt % crystalline propylene homopolymer; and b) from 90 to 10 wt % of a crystalline propylene copolymer, the copolymer comprising units derived from propylene and units derived from at least one other comonomer, preferably ethylene, wherein the wt % of the comonomer based on the total weight of the polymer is in the range of from 0.05 to 15. The polymer is prepared using a metallocene catalyst system comprising at least two metallocenes in a polymerization process that involves the sequential or parallel polymerization of the propylene homopolymer and copolymer. In one embodiment, the polymerization is conducted in slurry reactors connected in series with the propylene homopolymer being produced in the first reactor and the propylene cocopolymer being produced in the presence of the initially produced homopolymer.